Vitamin D is good for bone health – but only when taken with calcium supplements, a new report finds. And it’s too soon to say whether it helps prevent cancer.

The report, based on a review of vitamin D studies, found that daily doses of vitamin D between 300 and 1,100 IUs, combined with 500 to 1,200 mg of calcium were found to reduce fracture risk in seniors over the age of 65. The report by the government-backed U.S. Preventive Services Task Force (USPSTF) was published in the journal Annals of Internal Medicine.

“I think vitamin D likely has more benefits than we currently know and also little harm,” said Dr. Robert Graham, a vitamin D researcher and internist at Lenox Hill Hospital in New York City. But finding an acceptable dosage level is “a moving target.”

Previous research indicated that high dosages of vitamin D may reduce the risk of cancers, but more research is need, the USPSTF said. Dosing levels remain a concern because too much vitamin D can increase the risk of kidney stones.

Focusing on bone thickness in future studies could help nail down proper doses for different age groups, said report author Mei Chung of Tufts Medical Center in Boston.

According to the Institute of Medicine, adults are advised to get between 600 and 800 IU of vitamin D and between 1,000 to 1,200 mg of calcium per day.